Various packaging techniques have been used to build a load of unit products and subsequently wrap them for transportation, storage, containment and stabilization, protection and waterproofing. One system uses stretch wrapping machines to stretch, dispense and wrap stretch packaging material around a load. Stretch wrapping can be performed as an inline, automated packaging technique that dispenses and wraps packaging material in a stretch condition around a load on a pallet to cover and contain the load. Pallet stretch wrapping, whether accomplished by a turntable, rotating arm, vertical rotating ring, or horizontal rotating ring, typically covers the four vertical sides of the load with a stretchable film such as polyethylene film. In each of these arrangements, relative rotation is provided between the load and the packaging material dispenser to wrap packaging material about the sides of the load.
Stretch wrapping machines provide relative rotation between a stretch wrap packaging dispenser and a load either by driving the stretch wrap packaging dispenser around a stationary load or rotating the load on a turntable. Upon relative rotation, packaging material is wrapped on the load. Ring style stretch wrappers generally include a roll of packaging material mounted in a dispenser, which rotates about the load on a ring. Wrapping rings are categorized as vertical rings or horizontal rings. Vertical rings move vertically between an upper and lower position to wrap film around a load. In a vertical ring, as in turntable and rotating wrap arm apparatuses, the four vertical sides of the load are wrapped, along the height of the load. Horizontal rings are stationary and the load moves through the ring, usually on a conveyor, as the dispenser rotates around the load to wrap packaging material around the load. In the horizontal ring, the length of the load is wrapped. As the load moves through the ring and off the conveyor, the packaging material slides off the conveyor (surface supporting the load) and into contact with the load.
Historically, ring style wrappers have suffered from excessive film breaks and limitations on the amount of containment force applied to the load (as determined in part by the amount of pre-stretch used) due to erratic speed changes required to wrap “non-square” loads, such as narrow, tall loads, short, wide loads, and short, narrow loads. The non-square shape of such loads often results in the supply of excess packaging material during the wrapping cycle, during time periods in which the demand rate for packaging material by the load is exceeded by the supply rate of the packaging material by the dispenser. This leads to loosely wrapped loads. In addition, when the demand rate for packaging material by the load is greater than the supply rate of the packaging material by the dispenser, breakage of the packaging material may occur.
When stretch wrapping a typical rectangular load, the demand for packaging material varies, decreasing as the packaging material approaches contact with a corner of the load and increasing after contact with the corner of the load. When wrapping a tall, narrow load or a short load, the variation in the demand rate is even greater than in a typical rectangular load. In vertical rings, high speed rotating arms, and turntable apparatuses, the variation is caused by a difference between the length and the width of the load. In a horizontal ring apparatus, the variation is caused by a difference between the height of the load (distance above the conveyor) and the width of the load.
The amount of force, or pull, that the packaging material exhibits on the load determines how tightly and securely the load is wrapped. Conventionally, this force is controlled by controlling the feed or supply rate of the packaging material dispensed by the packaging material dispenser with respect to the demand rate of packaging material required by the load. Efforts have been made to supply the packaging material at a constant tension or at a supply rate that increases as the demand rate increases and decreases as the demand rate decreases. However, when variations in the demand rate are large, fluctuations between the feed and demand rates result in loose packaging of the load or breakage of the packaging material during wrapping.
The wrap force of all known commercially available pallet stretch wrapping is controlled by sensing changes in demand and attempting to alter supply of film such that relative constant film wrap force is maintained. With the invention of powered pre-stretching devices, sensing force and speed changes was immediately recognized to be critically important. This has been accomplished using feedback mechanisms typically linked to spring loaded dancer bars and electronic load cells. The changing force on the film caused by rotating a rectangular shaped load is transmitted back through the film to some type of sensing device which attempts to vary the speed of the motor driven pre-stretch dispenser to minimize the force change on the film incurred by the changing film demand. The passage of the corner causes the force on the film to increase. This increase force is typically transmitted back to an electronic load cell, spring-loaded dancer interconnected with a sensing means, or by speed change to a torque control device. After the corner is passed the force on the film reduces as the film demand decreases. This force or speed is transmitted back to some device that in turn reduces the film supply to attempt to maintain a relatively constant wrap force.
For example, U.S. Pat. No. 4,418,510 includes an embodiment that sets a pre-stretch roller speed to a reference speed faster or slower than the rotating load. This embodiment experienced no commercial success due the difficulty of practically achieving that process with market acceptable cost and satisfactory wrap performance. Accurately setting and maintaining the reference speeds with the disclosed embodiments proved problematic.
These concepts have proven themselves to be satisfactory for relatively lower rotation speeds where the response time of the sensing device and the physical inertia permit synchronous speed change with corner passage.
With the ever faster wrapping rates demanded by the industry, rotation speeds have increased significantly to a point where the concept of sensing demand change and altering supply speed is no longer effective. The delay of response has been observed to begin to move out of phase with rotation at approximately 20 RPM. The actual response time for the rotating mass of film roll and rollers approximating 100 lbs must shift from accelerate to decelerate eight times per revolution, which at 20 RPM is a shift more than every ½ sec.
Even more significant is the need to minimize the acceleration and deceleration times for these faster cycles. Initial acceleration must pull against the clamped film, which typically cannot stand a high force, especially the high force of rapid acceleration. Thus, acceleration cannot be maintained by the feedback mechanisms described above.
Film dispensers mounted on horizontally rotating rings present additional special issues concerning effectively wrapping at high speeds. All commercially available ring wrappers in use depend upon electrically powered motors to drive the pre-stretch film dispensers. The power for these motors must be transmitted to the rotating ring. This is typically done through electric slip rings mounted to the rotating ring with an electrical pick up finger mounted to the fixed frame. Alternately, others have attempted to charge a battery or run a generator during ring rotation. All of these devices suffer complexity, cost and maintenance issues. But even more importantly they add significant weight to the rotating ring which impacts its ability to accelerate and decelerate rapidly.
Film dispensers mounted on vertically rotating rings have the additional problem of gravity forces added to centrifugal forces of high-speed rotation. High-speed wrappers have therefore required expensive and very heavy two part bearings to support the film dispensers. The presence of the outer race on these bearings has made it possible to provide a belt drive to the pre-stretch dispenser. This drive is taken through a clutch type torque device to deliver the variable demand rate required for wrap force desired.
Due to the problems described above, use of high speed wrapping has been limited to relatively lower wrap forces and pre-stretch levels where the loss of control at high speeds does not produce undesirable film breaks.